1. Field of the Invention
This invention relates to a control method for improving the operational efficiency of a speed-controlled AC motor which is parallel-connected, together with a capacitive load, to the output of a current-controlled inverter for converting DC power into AC power, and to a control apparatus using this control method.
2. Discussion of Background
Various prior arts, relevant to the above control method or control apparatus, are known. Typical examples of the prior art are described in a paper entitled "Characteristic of Induction Motor Driven by Sinusoidal Output Current GTO Inverter" in National Convention Records of the Institute of Electrical Engineers of Japan, 1984, No. 582, and another example is the circuit shown in FIG. 1.
In the approach of the above paper, the inverter circuit is made up of self-extinguishing type semiconductor elements, such as gate turn-off thyristors (GTOs). The output current of the inverter is PMW-controlled to have a sinusoidal waveform. In this approach, the inverter circuit is controlled by forced commutation over the entire range of its operating speed. For this reason, power loss is increased in a snubber circuit for absorbing the switching surge current of the semiconductor elements. The result is a reduction in the operating efficiency of the inverter circuit.
The circuit of FIG. 1 is based on natural commutation. This approach, however, still entails many problems which need to be solved.
In FIG. 1, reference numeral 11 denotes an input terminal for the AC power source; 12, a rectifier; 13, a DC reactor; 14, an inverter circuit; 15, an induction motor serving as an inductive load; 16, a capacitive load such as a capacitor; 14.sub.2, a thyristor. In FIG. 1, the AC power supplied from input terminal 11 for the AC power source is converted into DC power by rectifier 12. The converted DC power is smoothed by DC reactor 13. Inverter circuit 14 converts the smoothed DC power back into AC power, and supplies it to induction motor 15 and capacitive load 16. The relation of the currents of induction motor 15 and capacitive load 16 is illustrated in FIG. 2. In this figure, load current IM of induction motor 15 contains active current component IP and delayed reactive current component II with power factor angle .theta.1. Current IC of capacitive load 16 is an advanced reactive current component. Therefore, if advanced reactive current component IC is larger than delayed reactive current component IL, inverter 14 supplies current II with advanced power factor angle .theta.2, as is shown in FIG. 4. When inverter 14 is operating to supply advanced current II and power factor .theta.2, at the commercial power supply frequency band (50 to 60 Hz), is in the range from 10.degree. to 20.degree., even if thyristors are used for switching the elements in the arms of inverter circuit 14, these thyristors can be load-commutated or commutated in a natural manner.
If it is possible to operate induction motor 15 in a speed-controllable manner while thyristers 14.sub.2 of inverter circuit 14 are load-commutated, the operating efficiency of the inverter circuit is remarkably improved as compared to that obtained when induction motor 15 operates at a variable speed during the forced commutation of inverter circuit 14. If the motor is operated in the above manner, the circuit arrangement of inverter 14 can be made simple, so that the circuit design of the inverter for high voltage operation becomes easy. Although a synchronous motor may be used as the motor serving as the load, an induction motor will be used in the following description.
The circuit system of FIG. 1 has the advantageous features as mentioned above, but still has problems, which will be described below.
In FIG. 2, the current IC of capacitive load 16 is given as: EQU current IC .varies. (Inverter output frequency).sup.2.
This relation has been employed for the reason that it is common practice that a ratio of the output frequency of the inverter to the output voltage is controlled to be at a fixed value, during the variable control of the speed of induction motor 15 by inverter circuit 14. With this relation, when the operating frequency of inverter 14 is decreased, the current IC of capacitive load 16 is remarkably reduced, but delayed reactive current component II of induction motor 15 is substantially constant, irrespective of the operating frequency. Therefore, if the output frequency of inverter circuit 14 changes widely, output current II of inverter circuit 14 cannot maintain prescribed advanced power factor angle .theta.2, and the load commutation of inverter circuit 14 becomes impossible.
One of the best ways to solve this problem is to increase the capacitance of capacitive load 16. This approach, however, entails the problem that if the capacitance of load 16 is increased, the required output capacity of inverter circuit 14, at the maximum output frequency of the inverter, correspondingly increases. For example, in order to operate the inverter at a frequency of 100% to 50% of the output frequency, the output capacity (kVA) of inverter circuit 14 requires 300% to 400% of the input capacity (kVA) of induction motor 15, under a fixed torque load.